1. Field of the Invention
The present invention relates to an electroluminescent material for forming electroluminescent cells, and to an electroluminescent cell.
2. Description of the Background Art
Use of organic materials as a hole transfer material and an electron transfer material for forming an electroluminescent cell has begun in recent years. Organic electroluminescent cells using such organic materials are being vigorously researched.
Organic materials that constitute such an organic electroluminescent cell are required to possess superior durability and high luminous efficiency.
Conventionally, as an organic material possessing a hole transfer capability, diamine derivatives which include low molecule organic materials such as aryl amine compounds, for example, N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-di(3-methylphenyl)-4,4xe2x80x2-diaminobiphenyl (hereinafter also referred to as xe2x80x9cTPDxe2x80x9d), and macromolecule organic materials such as polyvinyl carbazole are known.
However, the above-mentioned low molecular weight organic materials lack physical or thermal durability. A hole transfer layer based on such low molecular weight materials tends to deteriorate while the organic electroluminescent cells are in use or in storage.
On the other hand, macromolecule organic materials such as polyvinyl carbazole have a very high glass transition temperature (Tg) which provides a hole transfer layer with excellent durability. However, they present problems in actual application, requiring a very high drive voltage and yet yielding a low luminous efficiency due to a deficiency in hole transfer capability.
Although 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (hereinafter also referred to as xe2x80x9cPBDxe2x80x9d) is known as an electron transfer material, thin films made of PBD lack stability.
Japanese Patent Application Laid-open No. 255725/1997 discloses a copolymer of oxadiazole and aromatic diamine or vinyl carbazole. Although the copolymer of this patent is a bipolar-type and forms a monolayer electroluminescent cell, the resulting cell exhibits insufficient luminous brightness and durability.
In consideration of the above circumstances, a first object of the present invention is to provide an electroluminescent material having a high luminous efficiency and excellent durability.
A second object of the present invention is to provide an electroluminescent cell which exhibits a high luminous efficiency and excellent durability.
The electroluminescent material of the present invention comprises a unit (AB) which is formed by alternate copolymerization of hole transfer monomers and electron transfer monomers, and a unit (A) which is formed by the polymerization of hole transfer monomers, wherein the molar ratio of the total of the structural units originating from the hole transfer monomers and the structural units originating from the electron transfer monomers in the unit (AB), and the structural units originating from the hole transfer monomers in the unit (A) is from 50:50 to 5:95.
The electroluminescent material of the present invention may be a block copolymer comprising a block component consisting of the unit (AB) and a block component consisting of the unit (A).
Furthermore, the electroluminescent material of the present invention may be a resin composition comprising a copolymer consisting of the unit (AB) and a polymer consisting of the unit (A).
Moreover, the electroluminescent material of the present invention may be a resin composition comprising a block copolymer which comprises a block component consisting of the unit (AB) and a block component consisting of the unit (A), a copolymer consisting of the unit (AB), and a polymer consisting of the unit (A).
The electroluminescent cell of the present invention comprises an anode layer, an electroluminescent material layer made of the above-mentioned electroluminescent material, an electron transfer luminous layer, and a cathode layer. The present invention will now be described in more detail by way of embodiments.
Other objects, features and advantages of the invention will hereinafter become more readily apparent from the following description.